The Colorado River delivers agricultural runoff from across the American West. By the time the river reaches the international border, years of irrigation have concentrated the salinity of its water to levels that damage crops and soils. The 1944 Water Treaty with Mexico guaranteed Mexico 1.5 million acre-feet of Colorado River water annually, but the treaty said nothing about salt content. By the 1960s, Mexico was receiving water with salinity levels so high that they were causing crop failures in the Mexicali Valley.

Diplomatic pressure led to a 1973 agreement on water quality, and the 1974 Colorado River Basin Salinity Control Act authorized the federal response. The Yuma Desalting Plant would treat the most saline Colorado River water — agricultural drain water from the Wellton-Mohawk Irrigation and Drainage District — and return it to the river in a form clean enough to meet treaty obligations to Mexico.

The plant was designed to process 73 million gallons of water per day, making it the largest reverse osmosis desalination facility in the United States. Construction began in the mid-1980s. The plant was completed in 1992, at a cost that made it one of the most expensive water treatment projects in American history to that point.

The timing of completion created an immediate problem. By 1992, water from the Wellton-Mohawk District had been reduced through conservation measures, and the need for the plant's full capacity was less urgent than when it was designed. Running the plant at full capacity was expensive; the operational costs of desalinating 73 million gallons per day were substantial. The Bureau of Reclamation ran the plant briefly for testing and then, when the immediate diplomatic pressure eased, placed it in standby mode.

The plant has been operated periodically since — during drought years when river flows are low and salinity is high, it provides a useful supplement to treaty compliance — but it has never operated continuously at design capacity.

While the Yuma Desalting Plant was being built, engineers needed somewhere to send the brine concentrate and drainage water that the construction and testing phases produced. A drain canal — the Main Outlet Drain Extension, known as MODE — was constructed to carry this water to a former dry lakebed called the Santa Clara Slough, in Mexico near the Colorado River delta.

The unexpected consequence of routing water through the MODE canal was the creation of the Ciénega de Santa Clara: a wetland that grew to cover approximately 12,000 acres of the depleted Colorado River delta. The Ciénega became one of the largest remaining wetlands in the Sonoran Desert, a refuge for birds, fish, and other wildlife that had lost habitat as the delta dried up over the course of the twentieth century. The endangered Yuma clapper rail, nearly eliminated from the region, found significant habitat there.

The Ciénega de Santa Clara now depends on the continued flow of water through the MODE canal — water that comes partly from the desalting plant's operations and partly from agricultural drainage. It is, in ecological terms, the most important unintended consequence of a project that cost hundreds of millions of dollars and doesn't run most of the time.

The Yuma Desalting Plant sits near the confluence of the Gila River and the Colorado, a large facility visible from the air on the Arizona side of the border. Its reverse osmosis membranes and associated infrastructure represent the full capability of industrial-scale desalination, built at federal expense to meet an international obligation.

The plant exists in a perpetual state of readiness-without-operation, maintained so it can be turned on when conditions require it — when drought deepens, when river salinity spikes, when the treaty obligations cannot be met by other means. In the meantime, its most significant contribution to the region's ecology is the accidental wetland in Mexico that its construction drainage created: 12,000 acres of birds and reeds and water in a delta that the river itself abandoned.